The sweat chloride test is used to test for cystic fibrosis (CF) in symptomatic people. It is also used to help confirm or rule out a diagnosis of CF in those who have tested positive or indeterminate with other tests, including a screening sweat test, the immunoreactive trypsinogen test (IRT), the trypsin test, and/or the nasal (transepithelial) potential difference (NPD) test. It is not used to monitor CF since elevated levels do not correlate with the severity of the disease or its symptoms.
It may also be ordered in the case of people who have tested negative with a standard CF gene mutation panel but who are clinically still suspected of having CF. The CF gene mutation panel generally tests for the 23 most common CF gene mutations. If the panel is negative, then the person may not have CF or may have rare mutations not included in the screening panel. Sweat chloride testing may be diagnostic of CF even if the genetic mutations causing it cannot be identified. This testing may also be used to determine if additional mutation analysis is warranted.
A positive sweat chloride test indicates that there is a good chance that the person has CF. Positive sweat chloride tests should be repeated for verification and confirmed, wherever possible, by CF gene mutation panel testing.
A few people with CF will have a normal or inconclusive sweat chloride test and will have to be evaluated using other tests, or the diagnosis may be made on clinical grounds alone.
The sweat test may not always be useful in newborns. They may not produce enough sweat for a reliable diagnosis, so the test may have to wait until the baby is several weeks old. The sweat test may not need to be done if the baby has an elevated IRT and a positive CF gene mutation test.
The sweat test will not pick up carriers of cystic fibrosis. It takes two mutated copies of the CFTR gene to cause cystic fibrosis. Those who are only carriers do not have CF and will not have sweat chloride abnormalities.
The sweat chloride test should only be performed at specialized centers with experience with this kind of testing. Otherwise, problems in accuracy, including false negatives due to poor collection technique, can arise.
This article was last reviewed on December 11, 2012. | This article was last modified on December 11, 2012.
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